Note that as of January 1st, 2019, it is no longer possible to redeem AppSumo codes.
Yes, you can.
After you signup with the first AppSumo code at https://appsumo.webarxsecurity.com, you can redeem any other AppSumo codes on the billing page at https://portal.webarxsecurity.com/invoice/overview on the right side of the page "AppSumo Redeem Extra Coupon".
In case you did not signup through https://appsumo.webarxsecurity.com, you should contact us so we can upgrade your account manually.